Symbiosis

Sym biosistogether life

Symbionts: organisms involved

Host: larger organism, if there is one

Mutualism: both symbionts benefit

Commensalism: one symbiont receives benefit while neither harming nor helping the other in any significant way

Parasitism: one symbiont, called a parasite, benefits at the expense of the other, usually a host

Anthropomorphism in biological definitions?

Neutral

Commensalism

Mutualism

InterspecificCompetition

Predation

Parasitism

Direct Effect on Species 1

Direct Effect on Species 2

Type of Interaction

0 0

0

Examples

Ants and plants (greehouse examples): mutualism with a large host

Epiphytic plants (many orchids): commensalism with a large host

Parasitism: Giardia, Plasmodium

Parasites need to be distinguished between cases where a pathogen may lead to death and where there may be a balance in the host-parasite relationship. For example, there can be genetic balances in virulence and resistance that operate at the populations level. Myxomatosis and rabbits

N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi

A point of special interest is that the nitrogenase enzyme complex is highly sensitive to oxygen

Mutualism: Nitrogen fixing bacteria and Plants

In biological nitrogen fixation two moles of ammonia are produced from one mole of nitrogen gas, using 16 moles of ATP and a supply of electrons and protons (hydrogen ions):

This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an enzyme complex termed nitrogenase. This enzyme consists of two proteins - an iron protein and a molybdenum-iron protein.

The mutualism: plant gains nitrogen compounds, the bacterium gains carbohydrate and an environment with reduced oxygen

infectionthread

roothair

rootnodule

Soya bean Glycine maxand Rhizobium

Nodule formation 1

Roots emit chemical signals that attract Rhizobium bacteria. The bacteria emit signals that stimulate root hairs to elongate, and to form an infection thread by an invagination of the plasma membrane

Soya bean infection with Rhizobium

Nodule formation 2

The bacteria penetrate the root cortex within the infection thread. Plant cells start dividing and vesicles containing the bacteria, bacteriods, bud into the cells from the branching infection thread

Nodule formation 3

Growth continues in the affected regions of the cortex and pericycle and these fuse to form the nodule

Nodule formation 4

The nodule grows and vascular tissue connecting it to the plant’s xylem and phloem develops

TEM photomicrographs

http://www.sunderland.ac.uk/~es0man/tem2.htm

Small numbers of bacteria Large numbers of bacteria

7 days 12 days

Lens culinaris (lentil) root nodulation.

Leghaemoglobin

This shows as a pink colour when the active nitrogen-fixing nodules are cut open. Leghaemoglobin may regulate the supply of oxygen to the nodule tissues in the same way as haemoglobin regulates the supply of oxygen to mammalian tissues

Leghaemoglobin is found only in the nodules and is not produced by either the bacterium or the plant when grown alone.

Clover root nodules.

In symbiotic nitrogen-fixing organisms such as Rhizobium, root nodules can contain oxygen-scavenging molecules such as leghaemoglobin.,

G

Common beans, Phaseolus vulgaris, are poor fixers (< 50 lbs per acre) and fix less than their N needs. Maximum economic yield in New Mexico requires an additional 30-50 lbs of fertilizer N per acre. However, if beans are not nodulated, yields often remain low, regardless of the amount of nitrogen applied.

Some legumes, e.g., peanuts, cowpeas, soybeans, and faba beans are good nitrogen fixers, and will fix all of their N needs – up to 250 lbs of N per acre and are not usually fertilized.

There are many research programs attempting genetic improvement of nitrogen fixation, e.g., alfalfa. Genetic modification for tropical crops.

If large amounts of nitrogen are applied, the plant slows or shuts down the nitrogen fixation process.

Myco rrhizaeFungus Root

There are two major types:

ectotrophic mycorrhizae and

endotrophic mycorrhizae called vesicular-

arbuscular mycorrhizae because of the structures they produce inside

roots

Mycorrhizas are highly evolved, mutualistic associations between soil fungi and plant roots.

The host plant receives mineral nutrients while the fungus obtains photosynthetically derived carbon compounds.

Almost 80 percent of all terrestrial plants can form mycorrhizal associations.

Some fungi produce rhizomorphs containing specialised conducting hyphae, or sclerotia, which are resistant storage structures that survivein the soil

and then infect other plants.

Ectomycorrhizae

Fungal structures in soilAbsorptive hyphae

Mycellialstrand Scleridia

Mycorrhizalroot

Soil mycellium

Rhizomorphs

Mycorrhizal fungi produce a hyphal network in soils consisting of

individual strands of hyphae or relatively undifferentiated bundles of hyphae called mycelial strands.

Early stage of colonisation of pine short root by Pisolithus tinctorius. Hyphae (arrows) have contacted the root and are starting to proliferate on its surface near the apex (A).

SEM image showing the next stage of pine root colonisation by Pisolithus tinctorius. Mantle hyphae (arrows) have formed a dense covering on the root surface (arrows).

http://www.ffp.csiro.au/research/mycorrhiza/ecm.html

Example of ECM short roots (arrows) of birch (Betula

alleghaniensis), an angiosperm tree. The mycorrhizal

short roots are thicker than other laterals of the same

order due to the mycorrhizal infection.

Pinus radiata and Amanita muscaria ECM synthesised under sterile conditions. This association has highly branched short roots with many root tips (arrows).

Eucalyptus maculata and Astraeus pteridis association synthesised under sterile conditions with relatively unbranched ECM and attached mycelial strands (star).

Populus tremuloides ECM root cross section showing labyrinthine net hyphae (arrows) around elongated epidermal cells. This complex hyphal branching pattern is considered to increase the fungal surface area in contact with the root.

http://www.ffp.csiro.au/research/mycorrhiza/ecm.html

Hand section cleared and stained with Chlorazol black E and viewed with interference contrast microscopy

The network of hyphae in the soil is only connected to roots by the entry points that initiate mycorrhizal associations

Vesicular arbuscular mycorrhizae

(VAM)

Mycorrhizal root system washed carefully from coarse sand to reveal the intact network with external hyphae (arrow) with spores (S) produced by Glomus mosseae.http://www.ffp.csiro.au/research/mycorrhiza/ecm.html

Endotrophic

A colony of VAM refers to hyphal growth within a root resulting from the same external hyphae (1 or more connected entry points). These are also called infection units.

Epidermis

Hypodermis

CortexVesicle

Intracellularhyphae

AppressoriumAt entry point

Arbuscules

Intercellular hyphae in airchannel

The colony may produce arbuscules, exchange structures

Vesicles appear to be storage structures

Arbuscules (A) and convoluted hyphae (arrow) in the inner cortex of an Asarum canadense root. Arbuscules only form in the innermost cortex cell layer next to the endodermis in this species.

Vesicles (V) produced by a Glomus species in a leek root. This root also contains many intercellular hyphae. (Bar = 100 um)

http://www.ffp.csiro.au/research/mycorrhiza/ecm.html